Frothing a liquid involves supplying a gas to the liquid and mixing the liquid with the gas, for the purpose of obtaining a mass of bubbles in the liquid, which is commonly referred to as foam. A practical example of liquid to be frothed is milk. Especially in the field of making coffee and coffee specialties, there is a need for a device which is capable of frothing milk in a user-friendly manner. It is a well-known option to combine a frothing process of milk with a heating process of the milk, wherein air is supplied to the milk for obtaining foam, and wherein steam is supplied to the milk for increasing the temperature of the milk. Steam may also be used for realizing a pumping effect on the liquid to be frothed. To that end, the steam may be conducted through a restriction, wherein underpressure is created as the steam expands, which underpressure is suitable for causing suction of the liquid.
WO 2011/158171 A1 discloses a device for frothing a liquid which comprises a first space having an outlet for allowing content from the space to flow out of the space, means for supplying the liquid to be frothed to the first space, and a second space for allowing the liquid to mix with a frothing gas in order for a frothing process to take place. The second space is open to the first space, and has an inlet for receiving content from the first space, which is separated from the outlet of the first space by a free space for containing the frothing gas and for allowing for a supply of the frothing gas to the second space under the influence of a flow of liquid from the first space to the second space. Thus, at the position of this free space, when the device is operated and frothing gas is present, the frothing gas is allowed to flow into the second space and mix with the liquid to be frothed, wherein the gas can be made to flow under the influence of a flow of liquid to be frothed from the first space to the second space. In the second space, a swirling movement of the liquid and the frothing gas is induced for having the actual frothing process. This may be realized on the basis of having a tangential supply of the liquid and the frothing gas on a concavely curved surface, for example. It is noted that on the basis of a direction of the tangential supply of the liquid, a direction of the swirling movement of the liquid and the frothing gas, i.e. a direction of the swirling movement of the liquid froth during the frothing process, can be known. In particular, any skilled person can understand and predict what a rotation direction of the swirling movement will be, as the swirling movement is obtained on the basis of a continuation of a flow of the liquid which is bent as a result of contact to the concavely curved surface.
As a free inflow of frothing gas is possible in the configuration with the two spaces, namely at a position where one space ends and another space begins, as seen in a direction of a flow of liquid through the device, a certain intake of frothing gas can always be guaranteed, wherein disadvantageous effects which might influence the intake, particularly clogging effects, are avoided.
WO 2012/029019 A1 discloses a way of controlling the intake of frothing gas in the device as known from WO 2011/158171 A1. A tube that is provided for letting out the liquid froth is also used for letting out excess frothing gas. Furthermore, a number of requirements in respect of ratios of dimensions of components of the device are defined, which contribute to obtaining a high quality of liquid froth without excessive or insufficient foam formation or very coarse foam bubbles.
Although the type of device for frothing a liquid as known from WO 2011/158171 A1 and WO 2012/029019 A1 is practical, and capable of realizing excellent frothing results, there are some drawbacks related to that specific type. In the first place, the sound level associated with operation of the device is high when compared to other types of device for frothing a liquid, especially the types in which the frothing gas is supplied through a channel or the like. The reason is found in the fact that a high speed steam jet that travels through air or another frothing gas produces high frequency noise. This problem may be solved by providing appropriate shielding, but doing so requires extra components in the device and involves extra costs. In the second place, a small fraction of the liquid droplets may not be caught by the inlet of the second space. As a result, especially in case the liquid is milk, this may cause a need for cleaning effort. In the third place, in practice, the device is quite difficult to produce, because accuracy needs to be high in order to realize diameters of various components as required, and because it is advantageous to have taper angles at various positions of the device.